Short- and medium-distance migrant birds that return to the breeding grounds early can usually take better-quality territory leading to higher breeding success than of congeners arriving later. On the other hand, early breeders usually have to face severe weather conditions. 109 Black-headed Gulls which had begun laying in a breeding colony (N-Poland) earlier than the remaining 960 females lost 66.2% of eggs because of severely adverse weather. On 9 April 2012 ambient temperature fell to -4.6°C during the first two hours after sunrise (06:00–07:59) resulting in eggs cracking during the adults' first feeding bout after the night. Up to 1 mm wide cracks in eggshells were from 3 mm to 24 mm long; in some eggs the external shell membrane also broke. Pairs that lost eggs did not repeat broods. In this way in 2012 this colony suffered the greatest loss of eggs during its 15-years history. This event showed that even in the temperate climate, adverse weather, a consequence of the global climate change, does have negative impact on life history traits of birds.
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Vol. 63 • No. 3